Varietäten, Descendenz, Hybriden. 19 



The progressive variations are in their differences actually parallel 

 the differences existing betwcen many wild species of ferns and 

 flowcring plants. 



The main difference between these variations and those shown 

 by wild forms lies in the fact that these horticultural forms do not 

 possess adaptability to natural conditions. In most cases, with the 

 Variation, has come decreased vigor of growth. 



The cause or causes of these variations are undetermined. They 

 proceed as if from internal Stimulation. The improved cultural con- 

 ditions do not appear as causes, but rather as a means of preserving 

 forms which, under wild conditions, would be eliminated by natural 

 selection. 



The author adds an index of the Sports. He distinguishes here: 

 varieties showing increase in leaf division, varieties showing progres- 

 sive ruffling, varieties showing progressive dvvarting, varieties show- 

 ing dichotomy of pinnae and one variety of uncertain character. 



Jongmans. 



Emerson, R. A., The calculation of linkage intensities. 

 (American Natur. L. p. 411—520. 1916.) 



Two methods of estimating the intensity of linkage are in use. 

 One consists of crossing individuals heterozygous for two or more 

 linked genes with homozygous recessives. This is the more direct 

 method, because the gametic ratio, barring differential viability, is 

 exhibited directly by the zygotic frequencies. The other method 

 employs ordinary Fa-ratios derived from seifing Fj or breeding 

 together like F^-individuals. Here the gametic ratio can only be 

 inferred from the numerical relation of the zygotic classes. The 

 results may be disturbed not only by differential viability, as in 

 the first method, but also by selective fertilization, if that occurs, 

 and may often be materially influenced by chance in random 

 mating where the numbers are small. In fact, this method is so 

 undesirable that it would not ordinarily be used where the other 

 method is practicable. It is true, however, that the mechanical 

 difficulties of crossing certain plants are so great and the number 

 of seeds produced per flower so small, that often the ordinary F2 

 results are alone available. It is important therefore to have a 

 means of calculating gametic ratios from Fj zygotic numbers. 

 Since no direct formulae for calculating gametic ratios from 

 observed F^ data have heretofore been available, the writer has 

 attacked the problem in an indirect way. A series of Fg zygotic 

 ratios has first been calculated from a corresponding series of 

 gametic ratios. Next the observed F^ results have been compared 

 with the calculated series, the dosest fitting calculated ratio deter- 

 rained, and the corresponding gametic ratio taken as that respon- 

 sible for the observed F2 results. M. J. Sirks (Wageningen). 



Kraus, E. J., Somatic segregation. (Journ. of Heredity. VII. 

 p. 3-8. 1916) 



Vegetative variations are first of all of two distinct sorts: modi- 

 fications or fluctuations, which do not remain true when propagated 

 and subjected to varying conditions, and segregations or mutations 

 which may be propagated and expected to remain reasonably con- 

 stant under a wide ränge of conditions. In the past experiments 



